1. Field of the Invention
The present invention relates to a switching power supply device that inputs an alternating-current (AC) voltage from a commercial AC power source and generates a direct-current (DC) voltage.
2. Description of the Related Art
Conventionally, switching power supply devices have been known which supply voltage to control units such as a central processing unit (CPU) and drive units such as a motor in various types of electronic apparatuses. FIG. 7 illustrates the circuit configuration of a conventional switching power supply device. The switching power supply device includes a transformer 9 and a primary-side rectifying circuit. The transformer 9 includes a primary winding 9a, a secondary winding 9b, and an auxiliary winding 9c. The primary-side rectifying circuit includes a bridge rectifying circuit 2 and a smoothing capacitor 3, which rectify an AC voltage from a commercial AC power source 1 and supply the resulting DC voltage to the primary winding 9a of the transformer 9. The switching power supply device also includes a switching element 5 (in FIG. 7, a metal oxide semiconductor field-effect transistor (MOSFET) is used as an example) and an oscillation control circuit 6. The switching element 5 is connected in series to the primary winding 9c of the transformer 9. The oscillation control circuit 6 controls switching of the switching element 5. The switching power supply device also includes an auxiliary-side rectifying circuit and a starting resistor 4. The auxiliary-side rectifying circuit includes a rectifier diode 7 and a capacitor 8, which rectify the output of the auxiliary winding 9a of the transformer 9 and supply the resultant to a power supply terminal 36 of the oscillation control circuit 6. The starting resistor 4 is connected between the positive voltage output side of the primary-side rectifying circuit and the power supply terminal 36 of the oscillation control circuit 6.
A rectifier diode 10 and a capacitor 11 are connected to the secondary winding 9b of the transformer 9. The rectifier diode 10 and the capacitor 11 rectify the output from the secondary side of the transformer 9 to generate a DC voltage. Resistors 14 and 15, a shunt regulator 12, and a photocoupler 13 are connected to the secondary side of the transformer 9. The resistors 14 and 15 divide the output voltage. The shunt regulator 12 compares the voltage divided by the resistors 14 and 15 with an internal reference voltage, and amplifies the error for detection.
The output of the shunt regulator 12 is connected to the photocoupler 13 and transmitted to a feedback terminal 33 of the oscillation control circuit 6. The oscillation control circuit 6 performs pulse width modulation (PWM) control on the switching element 5 according to a current that flows through the phototransistor of the photocoupler 13. Such a circuit, constituting a feedback control loop, can control the output voltage of the secondary side to a target voltage.
Next, the activating operation of the switching power supply device illustrated in FIG. 7 will be described with reference to FIG. 8. An AC voltage is supplied from the commercial AC power source 1. The output from the rectifying circuit on the primary side of the transformer 9 initially charges the capacitor 8 through the starting resistor 4. Charging the capacitor 8 increases the voltage V1 that is supplied to the power supply terminal 36 of the oscillation control circuit 6. When the voltage V1 reaches an activation start voltage (here, set to 20 V) of the oscillation control circuit 6, the oscillation control circuit 6 operates to start the switching operation of the switching element 5 (period 1 in FIG. 8).
During the switching operation, the oscillation control circuit 6 consumes a current more than supplied from the starting resistor 4. This lowers the voltage V1 on the power supply terminal 36 (period 2 in FIG. 8). With the switching started, a voltage occurs in the auxiliary winding 9c of the transformer 9. The voltage is supplied to the power supply terminal 36 of the oscillation control circuit 6 through the auxiliary-side rectifying circuit, so that the voltage V1 rises in value again (period 3 in FIG. 8). After the oscillator control circuit 6 is powered by the starting resistor 4 for activation, the oscillator control circuit 6 is supplied with the voltage from the auxiliary-side rectifying circuit. It should be noted that the resistance of the starting resistor 4 and the capacitance of the capacitor 8 are set so that the voltage value of V1 remains at or above an operation stop voltage (here, set to 10 V) of the oscillation control circuit 6 in period 2 of FIG. 8. Japanese Patent Application Laid-Open No. 2003-333841 discusses a method of activating a power supply device using such a starting resistor 4.
The foregoing power supply device uses the starting resistor 4 to activate the oscillation control circuit 6. Such a power supply device may sometimes fail to properly activate the oscillation control circuit 6 if the oscillation control circuit 6 has a small hysteresis width between the activation start voltage and the operation stop voltage. To be more specific, suppose that the hysteresis width between the activation start voltage and the operation stop voltage is small as illustrated in FIG. 9. In such a case, even if the voltage V1 on the power supply terminal 36 of the oscillation control circuit 6 exceeds the activation start voltage to start a switching operation, a voltage drop after the activation brings down the voltage V1 to below the operation stop voltage to stop the switching operation. The power supply device subsequently repeats the cycle of activation and stop, resulting in being unable to activate.
To solve the problem of an activation failure due to such a small hysteresis width, measures such as increasing the capacitance of the smoothing capacitor 8 on the auxiliary side and reducing the resistance of the starting resistor 4 have been taken heretofore. However, the problems described below occur in such measure.
(1) Increasing the capacitance of the smoothing capacitor 8 increases the time for the voltage V1 on the power supply terminal 36 of the oscillation control circuit 6 to reach the activation start voltage, so that the time for the activation of the power supply device increases. In other word, it takes longer to activate the power supply device. Such a power supply device, if applied to an apparatus, may increase the waiting time for startup. (2) Reducing the resistance of the starting resistor 4 increases a power loss in the starting resistor 4, so that the power conversion efficiency of the power supply device decreases.